An ignition timing of a diesel engine is basically controlled on the basis of a fuel-injection timing. However, it is difficult to directly detect an actual ignition timing. Hence, whether the actual ignition timing correctly meets a desirable ignition timing is usually uncertain. The ignition timing of the diesel engine affects a nitrogen oxide (NOx) emission quantity in exhaust gas and fuel consumption. It is important to control the actual ignition timing to meet the desirable ignition timing for achieving an ideal balance between the NOx emission quantity and the fuel consumption, which have a trade-off relationship.
Japanese Unexamined Patent Publication No. 9-144583 discloses a technique including a vibration sensor (a so-called knock sensor) attached to a cylinder block of a diesel engine. The technique simply judges a timing, at which an amplitude of vibration of the vibration sensor has reached a certain level, as an ignition timing.
However, in fact, many vibrational components are present in addition to a vibrational component due to ignition and burning. It is difficult to completely separate the vibrational component due to the ignition and burning from the other vibrational components. In addition, vibration may vary among burning cycles. It is difficult to correctly judge the ignition timing by the method of the related art.